1. Field of the Invention
The present invention relates to a novel antitumor protein hybrid and a process for the preparation thereof. More particularly, the present invention relates to a novel protein hybrid, specifically useful as a remedy for malignant tumor, which has a moiety consisting of the antitumor immunoglobulin and a moiety substantially consisting of the fragment A of a diphtheria toxin, and a process for the preparation of the same.
2. Description of the Prior Art
Many drugs (anti-cancer agents) have hitherto been known to cure malignant tumor or cancer; however, since these drugs display considerable toxicity not only to tumor cells but also to normal cells, they can not be administered in doses large enough to perish tumor cells, which is their inherent demerit. To correct such demerit, several attempts have been made, wherein an anti-cancer agent or cytotoxic protein toxin is conjugated to a special carrier to make it attractive to tumor cells selectively. There exists an antitumor antibody (antitumor immunoglobulin), though very small in amount, in the blood or on the surface of tumor cells of a cancer patient. The antitumor antibody has hitherto been practically been obtained by first immunizing an animal with the cancer tissue and then absorbing the obtained antiserum with the normal tissue of a human being. Antitumor antibodies, whether autochthonous, allogenic, or xenogenic, do not necessarily display a cytotoxic effect against tumor cells; however, they have a common nature of binding to cancer cells with an extremely high selectivity. Antitumor antibodies have, therefore, been utilized as a carrier to make an anti-cancer agent or protein toxin bind to tumor cells selectively.
For instance, U.S. Pat. No. 4093607 discloses an antitumor drug comprising an antibody-drug conjugate in which an anti-cancer agent such as daunomycin, etc. is covalently bonded to the Fab' dimer of an antitumor immunoglobulin. This conjugate is a good one in respect of a fact that it binds the drug to a target tumor cell with some selectivity. But, in general, as the anti-cancer agent such as daunomycin, etc., when liberated from the conjugate prior to its entry to the cell, displays cytotoxicity not only against tumor cells but also against normal cells, it does not work satisfactorily in terms of efficacy to destroy tumor cells only. Moreover its cytotoxicity is not completely strong, either.
Studies have also been made as to the use of a diphtheria toxin, one of the protein toxins which are much stronger in cytotoxicity, in the place of an anti-cancer agent.
For instance, F. L. Moolten et al. report that they prepared a conjugate by conjugating a rabbit anti-SV40 antibody to a diphtheria toxin with glutaraldehyde as coupling agent and were able to protect hamsters challenged with SV40-transformed 3T3 cells by administering the conjugate to the hamsters (Journal of the National Cancer Institute, vol. 55, pp. 473-477, 1975).
P. E. Thorpe et al. report that a conjugate prepared by coupling a diphtheria toxin to an antilymphocytic antibody by means of chlorambucil greatly inhibited the protein synthesis of human lymphoblastoid cells, CLA4 (Nature, Vol. 271, pp. 752-754, 1978).
These studies show that a conjugate of diphtheria toxin and antibody displays toxicity against the tumor cells selectively. However, these conjugates, when used as an antitumor drug, are supposed to have some demerits as mentioned below. The first of the demerits is that the nonspecific toxicity of diphtheria toxin is not nullified. More particularly, the object of these methods is to concentrate diphtheria toxin on the surface of tumor cells by the aid of antitumor antibody; however, since the conjugate contains the whole molecule of diphtheria toxin in its composition, it is apt to bind to normal cell surface receptors for diphtheria toxin and display cytotoxicity against normal cells. The second of the demerits is found with the method of crosslinking the antibody with the diphtheria toxin. Many of the cross-linking agents such as glutaraldehyde, toluene diisocyanate, chlorambucil, etc. effect the cross-linking not only between the antibody and the toxin but also between the antibody and the antibody, and the toxin and the toxin, and moreover, they effect the formation of intramolecular bonds in the antibody and in the toxin molecule, thus causing the formation of undesirable products and decrease or loss of the antitumor activity.